The crystal structure of the mineral mertieite-II from the Kaarreoja River, Inari commune, Finnish Lapland, Finland, was refined to R₁ = 0.0222 (I) and 0.0228 (II) on the basis of X-ray diffraction data collected from two single crystals. The mineral is trigonal, space group is . The unit-cell parameters for the two crystals are determined as: a = 7.5172(3), c = 43.037(2) Å, V = 2106.1(2) ų (I); a = 7.5135(4), c = 43.003(3) Å, V = 2102.4(3) ų (II) with Z = 12. The occupancies of the Sb and As position in the structure were refined according to the Sb:As ratio of each crystal. The position As1 (Wyckoff 6b) is completely filled by As atoms. Any excess of As is distributed together with Sb on structural position M1 (Wyckoff 12c). The crystal chemical formulae are defined as Pd₈Sb_1.5(Sb_0.94As_0.06)As_0.5 (crystal I) and Pd₈Sb_1.5(Sb_0.88As_0.12)As_0.5 (crystal II). As → Sb substitution of up to 4.50 wt% of As does not affect the main structural topology of mertieite-II. Mertieite-II, Pd₈Sb_2.5As_0.5 and synthetic Pd₈Sb₃ are isotypic compounds. The crystal structures of synthetic Pd₈Sb₃ and natural mertieite-II, Pd₈Sb_2.5As_0.5, can be derived from the hexagonal close packing by filling additional layers between the close-packed 3⁶ layers. The structure consists of Sb and (Sb,As) triangular 3⁶, Pd triangular 3⁶ and Pd pentagonal-triangular 5.3³ layers. Stacking of Sb-, (Sb,As)- and Pd-nets along the z axis caused extension of the unit cell. It contains a total of 36 layers: six Sb1-nets, six (M1,As1)-nets, 12 Pd1-nets and 12 (Pd1,Pd3,Pd4)-nets.